1. Field of the Invention
This invention relates to an optical touch sensitive screen for an electronic device, such as a smartphone, tablet, laptop, e-reader or handheld gaming device.
2. Discussion of the Related Art
The provision of touch sensitive screens for electronic devices is becoming increasingly widespread. Conventional touch screen arrangements may comprise an LCD display surface and a resistive or capacitive overlay that is placed over the LCD display surface. In a resistive touchscreen, when pressure is applied to the LCD display surface, via the overlay, electronic signals representative of the relative location of such pressure, are generated by a controller IC and a device driver, or similar, in the host computing device interprets such signals as touch-based inputs, such as a key press or scroll action. Similarly, in a capacitive touchscreen, disturbance in the electric field, created in the overlay, is used to determine the touch action(s).
Resistive touch screens are particularly favoured for many applications in view of their low cost, low power consumption and stylus support. Resistive touch screens generally comprise conductive and resistive layers, separated by thin spaces. When a pointer, such as a finger or stylus, touches the screen and applies pressure thereto, contact is made between the layers, thereby effectively closing a switch and causing current to flow. A controller determines the current between layers to derive the position of the touch point.
However, there are a number of disadvantages associated with resistive touch screens. Firstly, as a result of the overlay, the screens are not fully transparent, and are generally not readable in direct sunlight. Secondly, such screens are subject to deterioration of the pressure sensitive layers, and also require periodic re-calibration. Furthermore, pressure is required to be applied for detection: thus, if insufficient pressure is applied by a pointer, it will go undetected. Finally, resistive touch screens are unable to discern that two or more pointers are touching the screen simultaneously, a function known as “multi-touch”.